Auditory brainstem correlates of basilar membrane nonlinearity in humans

A behavioral measure of the basilar membrane response can be obtained by comparing the growth in forward masking for maskers at, and well below, the signal frequency. Since the off-frequency masker is assumed to be processed linearly at the signal place, the difference in masking growth with level is thought to reflect the compressive response to the on-frequency masker. The present experiment used an electrophysiological analog of this technique, based on measurements of the latency of wave V of the auditory brainstem response elicited by a 4-kHz, 4-ms pure tone, presented at 65 dB SPL. Responses were obtained in quiet and in the presence of either an on-frequency (4 kHz) or an off-frequency (1.8 kHz) pure-tone forward masker. Wave V latency increased with masker level, although the increase was greater for the off-frequency masker than for the on-frequency masker, consistent with a more compressive response to the latter. Response functions generated from the data showed the characteristic shape, with a nearly linear response at lower levels and 4:1 compression at higher levels. However, the breakpoint between the linear region and the compressive region was at about 60 dB SPL, higher than expected on the basis of previous physiological and psychophysical measures.     

Auditory nerve responses to imposed displacements of the turtle basilar membrane

Impulse activity of single auditory nerve fibres was recorded in the isolated half-head of the turtle in response to displacements of a piezoelectric probe placed on the basilar membrane. The temporal pattern of firing in response to sinusoidal displacements of amplitude 0.1-1.0 nm r.m.s. at a fibre's characteristic frequency could be matched to that generated by low-level tonal stimuli delivered to the tympanum. Frequency-threshold curves for acoustic and mechanical stimuli had similar shapes and differed only at frequencies above 500 Hz where the middle ear should filter acoustic but not direct mechanical stimuli. Step displacements of the basilar membrane gave a transient periodic discharge which resembled the responses to acoustic clicks. Most fibres initially increased their firing rate for rarefaction clicks and displacements towards the scala tympani.     

Basilar membrane nonlinearity and its influence on auditory nerve rate-intensity functions

Previous papers have shown that the shapes of rate-intensity functions of auditory nerve fibres vary with spontaneous rate (Sachs and Abbas 1974; Sachs et al. 1989; Winter et al. 1990; Yates et al. 1990) and that the variation is due to the nonlinear properties of the basilar membrane. This paper examines the basilar membrane nonlinearity and provides a semi-quantitative explanation for it in terms of previous models (Zwicker 1979; Patuzzi et al. 1989) and an analogue model. It thereby provides explanations for the shapes of the basilar membrane input-output curves and for the way in which they vary with trauma. The shapes of the neural rate-intensity functions are quantified and shown to be consistent with the low-threshold data of Geisler et al. (1985). Several nonlinear properties of the cochlea, such as recruitment, are also interpreted.     

Auditory nerve responses to monophasic and biphasic electric stimuli

Charge-balanced, biphasic stimulus pulses are commonly used in implantable cochlear prostheses as they can be safely delivered to living tissue. However, monophasic stimuli are more efficient (i.e. producing lower thresholds) and likely provide more spatially selective excitation of nerve fibers. We examined the neural responses to monophasic, 'pseudomonophasic', and biphasic stimuli to better understand the inherent tradeoffs of these stimuli. Using guinea pig and cat animal models, we compared the auditory nerve responses to both 40 micros monophasic and 40 micros/phase biphasic stimuli using both electrically evoked compound action potential and single-fiber recordings. We also made comparisons using a computational model of the feline auditory nerve fiber. In all cases, our stimuli were cathodic monophasic and cathodic-first biphasic pulses. As expected, monophasic stimuli provided lower thresholds relative to biphasic stimuli. They also evoked responses with relatively longer latencies. We also examined responses to charge-balanced biphasic pulses composed of two phases of differing duration (i.e. pseudomonophasic stimuli). The first phase was fixed at 40 micros, while the second phase was systematically varied from 40 to 4000 micros. With a relatively long second phase, we hypothesized that these stimuli would provide some of the beneficial features of monophasic stimuli. Both the gross-potential and single-fiber data confirmed this and indicate that the largest incremental effects of changing the second-phase duration occur for durations less than 500 micros. Consideration of single-fiber data and computer simulations suggest that these results are consistent with the neural membrane acting as a leaky integrator. The computer simulations also suggest that the integrative properties at least partially account for the difference between our monophasic-biphasic results and previously published data. Our results apply to cathodic-leading stimuli; due to differing patterns of membrane depolarization, they may not be applicable to situations using anodic-leading stimuli. Finally, we observed differences between the guinea pig and cat response patterns. Compared to cats, guinea pigs produced smaller monophasic vs. biphasic threshold differences. This interspecies disparity may be due to differences in cochlear anatomy.     

Basilar membrane nonlinearity determines auditory nerve rate-intensity functions and cochlear dynamic range

In a previous paper (Winter et al., 1990) we demonstrated the existence of a new type of auditory-nerve rate-intensity function, the straight type, as well as a correlation between rate-level type, threshold and spontaneous rate. In this paper we now show that the variation in rate-intensity functions has its origin in the basilar membrane nonlinearity. Comparison of rate-intensity functions at characteristic frequency and at a tail-frequency show that the rate-intensity functions are identical at low firing rates and that the sloping-saturation and straight types deviate from the standard function only at higher firing rates. The frequencies at which the deviations occur, and the change from saturating to sloping-saturation or straight, are closely correlated with the characteristic frequency of the fibre. Using the tail-frequency rate-intensity function as a calibration, it is possible to derive the basilar membrane input-output function at characteristic frequency from the characteristic frequency rate-intensity function. The resulting derived basilar membrane input-output functions are of a simple form and agree well with published direct measurements of basilar membrane motion. They show that the wide dynamic range to which the cochlea responds, about 120 decibels, is compressed by the basilar membrane nonlinearity into a much smaller range of about 30-35 decibels. General characteristics of the derived basilar membrane input-output curves show features which agree well with psychoacoustic studies of loudness estimation.     

Time course of rate responses to two-tone stimuli in auditory nerve fibres in the guinea pig.

Peri-stimulus time histograms (PSTHs) were constructed from responses of auditory nerve fibres in anaesthetized guinea pigs. Acoustic stimuli consisted of pure tones, presented either as tone bursts, or in two-tone combinations in which a gated test tone was superimposed on a continuous excitatory tone at characteristic frequency (CF). The majority of the sample of fibres displayed two-tone rate suppression (2TRS). The suppression was either a monotonic or a non-monotonic function of the level of the superimposed test tone. Monotonic suppression of CF-driven rate occurred only for test tones at frequencies higher than CF, presented at levels up to the maximum available (approx. 100 dB SPL). For test tones below CF, 2TRS initially increased, then reverted towards excitation for higher levels of the test tone. Three levels were identified in non-monotonic, two-tone rate functions; (1) the threshold for rate suppression, (2) the maximally suppressing level and (3) the level (referred to as the balance point) at which average firing rate was restored to the background, CF-driven rate. PSTHs for two-tone responses obtained for test tone levels between the maximally-suppressing level and the balance point typically showed brief decrements (notches) in spike rate, at the onset and following the offset of the test tone. The latency, depth and duration of notches, however, depended on the level of the test tone, in a different manner for onset and offset. In some cases, without overt rate excitation above the probe-driven rate, the offset notch became more pronounced and of extended duration with increased level of the test tone, suggestive of adaptation to the test tone. Two-tone responses, in which rate exceeded the background, CF-driven rate, in general were preceded by a reduced onset notch and were followed by a longer-lasting depression of the background spike rate, typical of post-excitatory depression. Relative to responses obtained to the test tones presented alone, excitatory two-tone responses were of lower rate and were delayed by the onset notch. Onset notches sometimes preceded rate excitation in responses to single tones. Some features of the time course of rate suppression and excitation displayed in PSTHs for responses to one and two-tone stimuli seem inconsistent with current models of 2TRS.     

Shapes of rate-versus-level functions of primary auditory nerve fibres: test of the basilar membrane mechanical hypothesis.

Rate-versus level functions (RI functions) for characteristic frequency (CF) stimulation were measured from primary auditory nerve fibres from different spontaneous rate categories in the guinea pig cochlea. Attention was focussed on those fibres that showed clear breakpoints in their RI functions (sloping-saturation fibres). A statistical curve fitting procedure to an empirical equation was used to provide a quantitative estimate of the breakpoint position in individual fibres. It was found that, within the limits of reliability of the curve fitting procedure, the breakpoint position was the same in fibres from the same CF regions in any given animal. This result is consistent with the notion that the breakpoint position is determined by global basilar membrane mechanics and not by processes private to each nerve fibre. However, a subgroup of fibres not easily classifiable as sloping-saturation, showed features of their RI functions suggesting that factors other than basilar membrane mechanics could lead to fibre-to-fibre differences in rate-versus-level behaviour.     

Homodyne interferometer for basilar membrane measurements

Techniques available for measuring the mechanical response of the inner ear are compared. These include capacitive probe, M?ssbauer and interferometric methods. The theory of a homodyne interferometer utilized for inner ear measurements is given. Experimental apparatus built to test the interferometer performance is described. Experimental results show that the measuring system can detect vibrations as low as 3 X 10(-12) cm. Its frequency response is flat within 1 dB from 0.1 to 40 kHz. It has a linear dynamic range of over 90 dB. Immunity of the interferometer to various disturbances is demonstrated.     

Auditory responses to the envelopes of pseudorandom noise stimuli in humans

Averaged scalp potentials evoked by continuous pseudorandom noise can be cross-correlated with the evoking stimulus, yielding a cross-correlation function (CCF) which reflects neural phase-locking and is quite sensitive for low-frequency stimulus components [M.J. Wilson and R.A. Dobie (1987) Electroencephalogr. Clin. Neurophysiol. 66, 529-538]. However, for higher frequency signals, replicable CCFs can only be obtained at moderate to high intensities. Since auditory neurons also respond to envelopes of complex sounds, even for high-frequency carriers, we compared scalp responses evoked by band-limited complex sounds to the envelopes of these sounds; the resultant envelope cross-correlation functions (ECCFs) contained replicable response components primarily below 1,000 Hz, regardless of the evoking stimulus spectrum. ECCF thresholds for three octave-band stimuli (830-1,562, 1,611-3,125, and 3,174-6,201 Hz) were more sensitive than CCF thresholds (P = 0.006), averaging 35 dB spectrum level for 10 normal subjects. When stimuli with only odd harmonics were used, replicable odd-component scalp responses were seen only in the spectral range of the stimuli, while even-component responses (presumably to stimulus envelope) were seen only in low-passed scalp responses.     

Auditory brain stem responses of premature infants to bone-conducted stimuli: a feasibility study

The feasibility of bone conduction auditory brain stem response (ABR) audiometry in intensive care nursery neonates was investigated. Forty premature infants were tested with both air- and bone-conducted stimuli. Bone-conducted stimuli resulted in more identifiable ABRs and a greater number of subjects passing the hearing screening. The findings of this study suggest that bone conduction ABR audiometry is a feasible technique with premature infants. Due to the lower frequency composition of the bone-conducted click, it may be more effective than an air-conducted click when the immature cochlea is being evaluated.     

Auditory neurophonic responses to amplitude-modulated tones: transfer functions and forward masking

Two auditory neurophonic responses - one recorded from the scalp (frequency following response or FFR) and one from the auditory nerve (auditory nerve neurophonic or ANN) - were obtained following stimulation of the cat cochlea with amplitude-modulated (AM) high-frequency tones. The carrier frequencies varied between 2 and 30 kHz. The modulation frequencies varied between 400 and 3000 Hz. The AM responses were compared with pure-tone neurophonic responses. The AM response waveforms were found to have a similar spectral composition, similar rates of adaptation, and similar rates of recovery from forward masking as the comparable pure-tone responses. As with the pure-tone neurophonics, an unmodulated masking stimulus can produce prolonged depression of the probe response. The amount and duration of this depression is dependent upon the level and frequency of the masker. The frequency dependence of the depression is demonstrated by forward masked tuning curves which indicate that the AM responses arise from fiber populations which have restricted characteristic frequency distributions centered on the carrier frequency. Response amplitude as a function of stimulus level (I/O) functions, response amplitude as a function of carrier frequency (carrier transfer functions or CTF) and response amplitude as a function of modulation frequency (modulation transfer functions or MTF) were also measured. It was found that the I/O functions were saturating monotonic functions of stimulus intensity, CTFs were flat for carrier frequencies from 6 to 30 kHz, and MTFs were flat for modulation frequencies from 100 to 1500 Hz. These results are compared with similar data for single units and compound action potentials.     

Auditory brainstem evoked potential latency-intensity functions: A corrective algorithm

The dynamic changes in the latency of the components of the auditory brainstem evoked potentials (ABEP) were analyzed and correlated with the psychophysical magnitude estimates of the stimuli evoking the potentials. This study included the reanalysis of the results originally reported by Pratt and Sohmer (1977, Electroencephalogr. Clin. Neurol., 43, 802–812). this time with correction for asymptote of the latency-intensity functions. The results of reanalyzing latency-intensity power functions have yielded exponents that were very similar across components, closer to the exponent of magnitude estimates and accounting for a higher amount of variance for all of the ABEP components. This procedure may also prove useful for clinical evaluation of auditory function.     

Critical bandwidth and consonance: their operational definitions in relation to cochlear nonlinearity and combination tones

A recent paper (Greenwood, 1990) reviewed cochlear coordinates in several species in relation to empirical frequency-position functions (Greenwood, 1961b, 1974b), one of which well fits the Békésy-Skarstein human cochlear map (Békésy, 1960; Kringlebotn et al, 1979). This increased the independence of the human function from the psychoacoustic data originally used to construct it and encouraged a second assessment of the relations of similar psychoacoustically significant bandwidths to distance and position on the cochlear map. The companion paper (Greenwood, 1991, this issue), found that, among such bandwidths, 'classical' critical bandwidth, and also 'constant interval', estimates in man correspond to equal distances to a closer extent than generally recognized, and over large parts of the frequency range they conform also to an exponential function of distance, as do most of the ERB estimates. This correspondence to almost constant and similar distances facilitates, and forms a part of, an explanation of the operational definitions of critical bandwidth in different experiments. The present account recapitulates the basic explanation of critical bandwidth and consonance offered in Greenwood (1971, 1972b, 1973b, 1974b) and Greenwood et al. (1976): by adding schematic details to the earlier account of critical bandwidth measurements in pure tone masking (the masker-notch interval), two-tone masking, narrow-band masking, and two-tone dissonance-consonance judgements and by outlining its applicability to AM and Quasi-FM detection and to two-band (nominally notched-noise) masking experiments. The measured bandwidths derive from approximately uniform dimensions of traveling wave envelopes in the peak region and from the effects of the resulting spatial pattern of nonlinear interference among primary components. In this account, critical bandwidth in man corresponds to a distance of about 1 or 1.25 mm, depending upon the direction the interval projects from the stimulus frequency to which it is referenced. It is identified with the apical segment of the traveling wave displacement envelope, which in guinea pig and squirrel monkey appears to be about 2/3rds and 3/4ths of a millimeter, respectively and would be about 1.25 mm in man if these distances were scaled (Greenwood, 1962) among these three species (Greenwood, 1974b, 1977a). When reflected also in the basal direction, the upper end of the frequency interval, at a 1.065 mm distance, makes a total two-critical-band distance, which corresponds with the region of nonlinear input-output functions that extends in both directions from the envelope peak and hence also with the frequency-dispersive region of accelerated phase accumulation (Greenwood, 1974b, 1977a).(ABSTRACT TRUNCATED AT 400 WORDS)     

Auditory steady-state responses to multiple simultaneous stimuli in children with functional or sensorineural hearing loss

The goal of our study was to identify the role of auditory steady-state responses for hearing assessment in patients with functional hearing loss. The study design was to compare auditory steady-state response thresholds and standard pure-tone audiometry thresholds between patients with functional or sensorineural hearing loss. Subjects comprised 16 patients (24 ears) with functional hearing loss and 17 patients (24 ears) with sensorineural hearing loss. Differences and correlations between auditory steady-state response thresholds and standard pure-tone audiometry thresholds at 500, 1,000, 2,000 and 4,000 Hz were evaluated. In children with functional hearing loss, pure-tone audiometry thresholds and auditory steady-state response thresholds were significantly different at all frequencies and were not significantly correlated. In patients with sensorineural hearing loss, pure-tone audiometry thresholds and auditory steady-state response thresholds did not differ significantly at any frequencies and were significantly correlated. Auditory steady-state responses may have principal role in the assessment of auditory brainstem acuity, particularly at low frequencies in patients with functional hearing loss.     

Auditory steady-state responses to MM and exponential envelope AM2/FM stimuli in normal-hearing adults

The present study utilized a commercially available multiple auditory steady-state response (ASSR) system to test normal hearing adults (n=55). The primary objective was to evaluate the impact of the mixed modulation (MM) and the novel proposed exponential AM²/FM stimuli on the signal-to-noise ratio (SNR) and threshold estimation accuracy, through a within-subject comparison. The second aim was to establish a normative database for both stimulus types. The results demonstrated that the AM²/FM and MM stimulus had a similar effect on the SNR, whereas the ASSR threshold results revealed that the AM²/FM produced better thresholds than the MM stimulus for the 500, 1000, and 4000 Hz carrier frequency. The mean difference scores to tones of 500, 1000, 2000, and 4000 Hz were for the MM stimulus: 20±12, 14±9, 10±8, and 12±8 dB; and for the AM²/FM stimulus: 18±13, 12±8, 11±8, and 10±8 dB, respectively. The current research confirms that the AM²/FM stimulus can be used efficiently to test normal hearing adults.